The present invention relates to improvements made to gyroscopic measuring apparatus with a mechanical resonator, comprising at least four identical, parallel vibrating beams integral with a common base and having the same natural frequency, each beam supporting piezoelectric elements for excitation purposes and for detecting vibration of the beam.
A gyroscopic measuring apparatus of this type is described in document FR-A-2 692 349. In this known gyrometer, the mechanical resonator comprises four parallel beams which are supported by a solid base in the form of a four-sided plate (square or rectangular in particular), the beams being disposed in the corner regions of the base.
Such resonators are made from a monolithic, rectangular, parallelepipedic metal bar, the external dimensions of which, in cross section, enclose the four beams. This bar is then machined to form two longitudinal slits perpendicular to one another, separating the four beams of the resonator. These slits must be machined with extreme precision, in terms of both direction and dimension (slit width e0), so that the beams thus delimited are strictly identical to one another and in practice as identical to one another as possible: any discrepancy in the dimensions destroys the symmetry of the device and causes vibrations which are transmitted to the exterior, through the support structure, and which upset the reliability of the gyrometer.
By way of example, in early gyrometers in which the beams were 60 mm in length, the width e0 of the slits to be machined were in the order of 2 mm; in modern gyrometers, the length of the beams has been reduced to 38 mm and the width e0 is in the order of 1 mm; in future gyrometers, it is desirable for the length of the beams to be reduced to approximately 15 mm, which would mean a value e0 in the order of 0.5 mm to an accuracy of within more than 0.1 xcexcm. Not only is it impossible to produce 0.5 mm slits economically, given current mechanical machining techniques (milling), it is practically impossible to respect the tolerance of 0.1 xcexcm across the entire length of the slit. Using different machining techniques (laser, in particular) would make manufacturing costs too high.
The gyroscopic measuring apparatus described in the aforementioned document also has a disadvantage inherent in its structure. In effect, the mechanical resonator with four vibrating beams should, in theory, correspond to a double tuning fork, i.e. in terms of vibration, only two diagonally opposed beams should be coupled. The very fact that the four beams are integral with a same solid base in the form of a plate, the shape of which conforms to the external contour of the four beams, means that this base couples all the beams to one another: in other words, not only are the beams coupled in diagonally opposite pairs (as desired), adjacent beams are also coupled in pairs (parasitic and not desired), which is the equivalent of six tuning forks (four of which are not desired). As a result, it is very difficult to balance such a device, to the degree that it is impossible to obtain the perfect balance that would be desirable (notwithstanding the problems inherent in the manufacturing tolerances).
Finally, another point which needs to be taken into consideration is the pressing demand on the part of users for gyrometers which are as compact as possible, making them easier to house in restricted and encumbered spaces, in addition to reducing the cost of using (which may be once only) the missiles to which they are fitted (guided warheads for example).
Recently, progress has been made in reducing the size of the mechanical resonator, since it has been possible to reduce the length of the vibrating beams from 60 mm to 38 mm. However, it is extremely desirable to reduce this length further and current thinking is now tending towards vibrating beams of a length in the order of 15 mm. Reducing the length of the vibrating beams by half will in effect double the resonance frequency, which, with conventional resonator designs (notwithstanding the manufacturing difficulties explained above), would lead to resonance frequencies in the order of 14 kHz, which is not acceptable.
There is an additional difficulty which should be pointed out, linked to the known structure of the mechanical resonator, which is due to the proximity (for example a difference in the order of 140 Hz) of the resonance modes to the operating mode (for example 7 kHz), which exacerbates filtering problems in the mode used.
Accordingly, the underlying objective of the invention is to propose an improved design of a mechanical resonator that will enable a gyroscopic measuring apparatus to be made which best meets the various practical requirements whilst eliminating the disadvantages of existing gyrometers and whilst enabling more compact resonators to be made under acceptable economic conditions.
To these ends, the invention proposes a gyroscopic measuring apparatus with a mechanical resonator comprising at least four identical, parallel vibrating beams integral with a common base and having the same natural frequency, each beam supporting piezoelectric elements for excitation purposes and for detecting vibration of the beam, which gyroscopic measuring apparatus, being designed as proposed by the invention, is characterised in that the base is cruciform and in that the beams are respectively disposed at the ends of the branches of the cross formed by the base.
A mechanical resonator of this design can essentially be likened to a double tuning fork, i.e. a resonator with two mutually transverse tuning forks formed by the two pairs of opposite beams. The common part of the two tuning forks is reduced to only the intersecting volume of the two branches of the cross formed by the base and this single volume is considerably smaller than that of the plate-shaped base used in existing resonators. The two tuning forks are effectively decoupled from one another and parasitic vibration modes are suppressed by one order (third order instead of second order as with the existing resonators): the device of the invention therefore comes close, with a good approximation, to a structure comprising two mutually transverse tuning forks.
Furthermore, the four beams, which are relatively spaced apart from one another, can be more easily machined than the existing devices, making them less expensive to manufacture than the existing devices where the beams are closer together.
In view of the specific geometry which can be used for the layout of the four vibrating beams, a considerable shortening of these beams becomes conceivablexe2x80x94and in particular their length can be reduced to a dimension in the order of 15 mm instead of the 38 mm of the beams used in existing devicesxe2x80x94whilst substantially conserving the same resonance frequency in the order of 7 kHz. Accordingly, it will then be possible to make smaller gyrometers than the currently known gyrometers, as desired by those who use them in numerous applications; alternatively, if using vibrating beams of the same dimensions as previously used, it becomes possible to reduce the resonance frequency (which may be reduced to 4500 Hz, for example), which is useful in terms of the design and operation of the electronics accompanying the gyroscopic measuring apparatus.
In one practical embodiment, which is easy and less expensive to machine, the branches of the cross formed by the base are substantially perpendicular to one another and are of identical lengths. This also makes it much easier to get the geometric balance of the structure right, which in turn makes it much easier to improve efficiency.
By preference, the branches of the cross are also all of the same width, which is substantially equal to the corresponding dimension of the beams. The height of the branches of the cross may be the same as the width of these branches (branches of a substantially square section) or may be different from this width, either greater or smaller. The choice of this height, which determines the choice of surface area of the cross section of the branches and hence their inertia, enables the value of the resonance frequency of the beams to be determined.
If the mechanical resonator has an embedded mounting foot which is integral with the base and extends parallel with the beams, it is desirable for the cross section of the foot to be disposed, at most, within the intersecting cross section of the branches of the cross formed by the base. This will make it easier to keep the foot more effectively decoupled from the vibrating elements.
In a first possible embodiment, the foot, relative to the base, extends remotely from the beams, in what is an otherwise normal layout. However, in view of the mutual spacing of the vibrating beams, it is also conceivable, in a second possible embodiment, for the foot, relative to the base, to extend on the same side as the beams and between them; the total length of the mechanical resonator will then correspond to the length of the vibrating beams plus only the thickness of the cross-shaped base. The resultant gain is a considerable fraction of the length of the device; this length, which may be in the order of 15 mm in devices with a projecting foot such as made possible by the invention, may be reduced even further to approximately 10 mm in devices where the foot is engaged between the beams: under these conditions, it becomes possible to design mechanical resonators with four vibrating beams which, encapsulated in a housing, will have overall external dimensions in the order of 14 mm in diameter and 10 mm in height for a resonance frequency in the order of 6 kHz.
In one example of a practical embodiment, and again with a view to simplifying the machining process as far as possible, it is of advantage if the lateral faces of the beams are parallel with and perpendicular to the branches of the cross formed by the base. This being the case, the four planes of the external faces of the four beams will then respectively describe a virtual square in vertical projection.
Finally, orifices are advantageously provided in each beam and/or recesses are hollowed into the faces of each beam in order simultaneously to adjust the resonance frequency of the beam and balance the beam so as to at least reduce, even eliminate, vibration in the foot: this double result, in the case of the vibrating device proposed by the invention, is achieved by this piercing/recessing work due to the fact that it effectively decouples the two tuning forks.
In the final analysis, the features proposed by the invention enable a gyroscopic measuring apparatus with a mechanical resonator to be obtained, which is small and therefore perfectly meets the aspirations of users; which is efficient and can achieve a performance in the order of 0.1 E/s, for example; which is robust and can therefore be used under difficult conditions, particularly under high acceleration conditions (fitted to guided warheads, for example).